The use of single-mode optical fibres in fibre-to-the-premises (FTTP) applications, including fibre-to-the-home (FTTH) and fibre-to-the-building (FTTB) applications, generally require low bending loss of optical signals transmitted through the fibres, also under stringent installation constraints that may impose tight bend radii, e.g., due to sharp cornering in buildings or compression of optical fibres. In particular, cabling and hardware applications aimed to miniaturize passive field equipment, e.g. local convergence cabinets or storage boxes, and the development of multi-dwelling units (MDUs) require fibre designs with superior bending capabilities. In addition, coarse wavelength division multiplexing systems (CWDM) and passive optical network (PON) systems may also need employment of bend-resistant optical fibres.
In order to standardize the bending performance of optical fibres particularly suited for FTTP applications, the ITU-T (International Telecommunications Union, ITU Telecommunication Sector) has developed recommendations G.657 that defines single-mode fibres with enhanced bending performance.
To conform to international standards, besides bending capabilities, fibre performance is evaluated also on other relevant optical parameters such as the cable cut-off wavelength and the mode field diameter (MFD). Recommended values for the MFD and for the cable cut-off wavelength are given in the ITU-T G.652 standard. A parameter that can be useful for finding a compromise between the MFD, the cut-off wavelength and bending losses is the so-called MAC number, which is the ratio of the MFD and the cut-off wavelength.
Optical fibres having an inner cladding region with depressed refractive index have been tailored to provide single-mode transmission and robust bend-resistance. The index depression of the inner cladding region is achieved, for example, by doping with fluorine a region surrounding the core region of the preform for production of the optical fibre.
EP 1785754 discloses an optical transmission fibre adhering to the G.652 standard for applications in FTTH or FTTC (fibre to the curb). The described fibre comprises a central core having an index difference Δn1 with an outer optical cladding; a first inner cladding having an index difference Δn2 with the outer cladding; a second, buried, inner cladding having an index difference Δn3 with the outer cladding of less than −3·10−3, and containing germanium in a weight concentration of between 0.5% and 7%. The preform of the fibre is produced by PCVD (Plasma Chemical Vapour Deposition), which is said to allow a high number of highly reactive fluorine ions and thus to add germanium to the reaction and nonetheless to obtain a buried inner cladding.
EP 1845399 relates to an optical transmission fibre having a refractive index profile with a first highly depressed section and a weakly depressed second section. The document states that with such a structure it is possible to effectively reduce the bending losses at constant MAC while strongly minimising the leakage modes of a higher order.
In U.S. Pat. No. 7,164,835, an optical fibre that exhibits a low loss due to bending and good connectivity with a common transmission optical fibre is described. The optical fibre comprises a core provided at a centre; a first cladding layer provided on a periphery of the core; a second cladding layer provided on a periphery of the first cladding layer; and a third cladding layer provided on a periphery of the second cladding layer, wherein a maximum refractive index of the core is greater than any of maximum refractive indices of the first cladding layer, the second cladding layer, and the third cladding layer, and the maximum refractive index of the second cladding layer is smaller than any of the maximum refractive indices of the first cladding layer and the third cladding layer, a value of a2/a1 is not less than 2.5 and not more than 4.5 when a radius of the core is a1 and a radius of an outer periphery of the first cladding layer is a2, and a relative refractive index difference of the core with respect to a refractive index of the third cladding layer is not less than 0.20 and not more than 0.70%.
EP 1788411 describes a single-mode optical fibre with a very small bending loss while conforming to the characteristics defined under ITU-T G. 652, such as chromatic dispersion, zero dispersion slope and cut-off wavelength. The fibre includes: a central core that has a radius r1 and a refractive index n1; an inner cladding that is provided around the outer circumference of the central core and has a radius r2 and a refractive index n2; a trench portion that is provided around the outer circumference of the inner cladding and has a radius r3 and a refractive index n3; and an outer cladding that is provided around the outer circumference of the trench portion and has a radius r4 and a refractive index n4, with a refractive index profile in which the refractive indexes of the individual portions satisfy n1>n4≧n2≧n3. With reference to the refractive index n4 of the outer cladding, a relative refractive index difference Δ1 of the central core, a relative refractive index difference Δ2 of the inner cladding, and a relative refractive index difference Δ3 of the trench portion satisfy the following relations: 0.40%≦Δ1≦0.85%, −0.20%≦Δ2≦0.00 and −1.0%<Δ3<Δ2.